Add a dbus method to find all the uid in a query without pagination

This is needed by the journal to implement the multi operations
feature. Without this, when the user "select all", the model is
transversed and query the datastore several times, depending
on the content of the journal and the page size (10).

Signed-off-by: Gonzalo Odiard <gonzalo@laptop.org>
Pep8 and pyflakes fixes on datastore.py

Signed-off-by: Gonzalo Odiard <gonzalo@laptop.org>